Method and apparatus to generate and monitor optical signals and control power levels thereof in a planar lightwave circuit

ABSTRACT

Embodiments of an optical detection apparatus are disclosed which may include one or more of a waveguide, a trench formed in the waveguide, a reflective surface, and a photodetector. The waveguide may be formed in a semiconductor substrate to propagate an optical signal received at a first end of the waveguide. The trench may also be formed in the waveguide having a first sidewall and a second sidewall, the first and second sidewalls forming first and second angles with the waveguide&#39;s propagation direction. The second sidewall may include a reflective surface formed thereon. The photodetector may be configured to receive an optical signal propagated in the waveguide, through the first sidewall and reflected from the reflective surface on the second sidewall.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 10/357,630,filed on Feb. 3, 2003, which has been allowed.

FIELD OF THE INVENTION

Embodiments of the invention relate generally to optical communicationsystems and more specifically but not exclusively to multi-wavelengthoptical signal generators for use in optical communication systems.

BACKGROUND INFORMATION

Optical signal generators (e.g., lasers) are widely used in opticaltransmitters in wavelength division multiplexed (“WDM”) opticalcommunication systems. Some optical signal generators use a distributedfeed-back (DFB) laser for each channel of the WDM system. The opticalsignals generated by the multiple DFB lasers are then combined usingelements such as arrayed waveguide grating based multiplexer or anyother multiplexer. However, because a separate DFB laser is used foreach channel, the optical transmitters tend to have increased complexityand cost. Further, the output wavelength of a DFB laser is relativelysensitive to temperature changes (i.e., thermal wavelength drift). Forexample, applications using DFB lasers need to provide special attentionto wavelength stability over the desired temperature range, therebyincreasing complexity and cost. Thus, reduction of this temperaturedependency is important task on its own merits.

In addition, the optical transmitters typically require circuitry tomonitor the power of the optical signal of each channel of the WDMsystem. This power monitoring circuitry is generally separate from theDFB laser devices (i.e., discrete), increasing the complexity and costsof fabricating the optical transmitters.

Still further, in many WDM applications, the power levels of the opticalsignals (of the various WDM channels) are equalized. Some approaches useseparate attenuator circuits (e.g., thermo-optic Mach Zendher devices)to equalize the power between channels. Again, such circuitry tends toincrease the complexity and cost of fabricating optical transmitters.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a block diagram illustrating an optical transmitterimplemented on a planar lightwave circuit (PLC), according to oneembodiment of the present invention.

FIG. 2 is a block diagram illustrating an external cavity laser arrayfor use in the optical transmitter of FIG. 1, according to oneembodiment of the present invention.

FIG. 3 is a block diagram illustrating an implementation of thetrench-based detector array of FIG. 1, according to one embodiment ofthe present invention.

FIG. 4 is a block diagram illustrating a photodetector and asymmetrictrench implementing an optical detector of the detector array of FIG. 3,according to one embodiment of the present invention.

FIG. 5 is a diagram illustrating a cross section of a PLC showing animplementation of the photodetector and trench of FIG. 4, according toone embodiment of the present invention.

FIG. 5A is a diagram illustrating a cross section of a PLC showing animplementation of the photodetector and trench of FIG. 4 and the powercontroller of FIG. 1, according to another embodiment of the presentinvention.

FIG. 6 is a flow diagram illustrating the operational flow of theoptical transmitter in controlling the optical power of each channel ofthe optical transmitter's output, according to one embodiment of thepresent invention.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known circuits, structures, and techniques have not been shown indetail in order to not obscure the understanding of this description.These embodiments are described in sufficient detail to enable those ofordinary skill in the art to practice the invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the invention is defined only by the appended claims.

FIG. 1 illustrates an optical transmitter 100 implemented on a planarlightwave circuit (PLC) 101, according to one embodiment of the presentinvention. In some embodiments, PLC 101 can have circuitry in additionto optical transmitter 100.

In this embodiment, optical transmitter 100 includes a laser array 110,a trench-based detector array 120, an arrayed waveguide grating (AWG)130, a grating array 140, and a power controller 150. In someembodiments, power controller 150 is not integrated on PLC 101, asindicated by dashed lines in FIG. 1.

This embodiment of optical transmitter 100 is interconnected as follows.Laser array 110 has N optical signal output waveguides 160 ₁-160 _(N)each having a corresponding grating of grating array 140 formed in theseoutput waveguides. Thus, in this embodiment, grating array 140 has Ngratings. As will be described below in more detail, each grating ofgrating array 140 defines a wavelength for a channel of an N-channel WDMsystem.

Detector array 120 has N optical input ports connected to correspondingwaveguides of waveguides 160 ₁-160 _(N). Detector array 120 also has Noutput ports, each connected to a corresponding waveguide of waveguides170 ₁-170 _(N). In this embodiment, detector array also has anelectrical output port that is connected to an electrical input port ofpower controller 150 via a line 191. Power controller 150 also has anelectrical output port, which is connected to an electrical input portof laser array 110 via a line 192.

AWG 130 has N input waveguides connected to a corresponding waveguide ofwaveguides 170 ₁-170 _(N). AWG 130 is configured to combine all of theoptical signals received at its input ports and output them via awaveguide 180.

In an alternative embodiment, laser array 110 may have a second set ofoutput waveguides (e.g., on the opposite side of the laser gain blocksto propagate light passing through the laser gain block reflectors).Detector array 120 may be connected to these output waveguides ratherthan in the “primary” output ports of laser array 110.

In operation, laser array 110 and grating array 140 form an array ofexternal cavity lasers providing optical output signals with N differentwavelengths (i.e., a wavelength for each channel of an N-channel WDMsystem). These external cavity lasers are described below in more detailin conjunction with FIG. 2.

Detector array 120, in this embodiment, is used to monitor the power ofeach of the N optical signals generated by the external cavity lasersformed by laser array 110 and grating array 140. Detector array 120 isdescribed below in more detail in conjunction with FIGS. 3 and 4. Insome embodiments, different power monitoring circuitry can be used withthe external cavity lasers formed by laser array 110 and grating array140. Still further, in some embodiments, different optical signalsources can be used with detector array 120.

Detector array 120 provides signals to power controller 150 indicativeof the power of each the N optical signals. The interconnection betweendetector array 120 and power controller 150 is described below inconjunction with FIGS. 5 and 5A.

Power controller 150 can then provide control signals to laser array 110to adjust the power of each of the N optical signals as desired for theapplication. One embodiment of this operation is described below inconjunction with FIG. 6. In addition, power controller 150 can include alaser drive circuit (or other circuitry) to directly modulate theoptical signals output by laser array 110 (e.g., on-off keying (OOK)).The N optical signals are then multiplexed and/or combined by AWG 130.

These embodiments of optical transmitter 100 can provide severaladvantages. For example, in embodiments that use the external cavitylasers formed by laser array 110 and grating array 140, the externalcavity lasers can be implemented using substantially identical lasergain blocks (with the wavelengths being defined by grating array 140),rather than multiple lasers of different wavelengths. Further, the lasergain blocks can be fabricated as a “laser bar” (i.e., multiple blockscleaved as a “bar” from a wafer) rather than being singulated. Anotheradvantage of using external cavity lasers with laser bar is thatintegration and positioning/alignment of the laser bar becomes a lessstringent process compared to that of separate laser gain modules (inwhich each module is aligned with its corresponding waveguide with aspecified accuracy. Other advantages of the external cavity lasers aredescribed below in conjunction with FIG. 2.

Another advantage, in embodiments that use detector array 120, is thatdetector array 120 can be easily integrated into PLC 101 due to itstrench-based design. Thus, the use of detector array 120 canadvantageously reduce the size and costs of fabricating opticaltransmitter 100.

In addition, in embodiments using detector array 120, the feedbackcontrol of the laser source (i.e., laser array 110) via detector array120 and controller 150 can eliminate the need for separate variableattenuators that are typically located downstream of the laser source toequalize power between channels. This approach can simplify the designand can reduce size and costs. This approach is different from somecurrent systems based on DFB or FP (Fabry-Perot) lasers in whichfeedback is used only to stabilize the wavelength power withinrelatively small range. However, some embodiments using detector array120 can provide wavelength stabilization as well as power equalization.

FIG. 2 illustrates an external cavity laser array 200 implemented usinglaser array 110 (FIG. 1) and grating array 140 (FIG. 1), according toone embodiment of the present invention. Detector array 120 (FIG. 1) andpower controller 150 (FIG. 1) are omitted from FIG. 2 to promoteclarity.

This embodiment of external cavity laser array 200 includes laser gainblocks 210 ₁-210 _(N), which are part of laser array 110. In thisembodiment, laser gain blocks 210 ₁-210 _(N) are substantiallyidentical, and each has a reflective surface serving as one mirror of anexternal cavity. In one embodiment, laser gain blocks 210 ₁-210 _(N) areimplemented using the laser diodes of an array of edge emitting laserdevices. In other embodiments, laser gain blocks 210 ₁-210 _(N) areimplemented using an array of vertical cavity surface emitting lasers(VCSELs).

This embodiment of external cavity laser array 200 also includesgratings 220 ₁-220 _(N), which are part of grating array 140. Gratings220 ₁-220 _(N) serve as the other mirrors for the array of externalcavity lasers. In this embodiment, gratings 220 ₁-220 _(N) have areflectivity of about 60%, although in other embodiments a reflectivityranging from about 50% to 70% may be used. Each of gratings 220 ₁-220_(N) are designed for the corresponding block of laser gain blocks 210₁-210 _(N) according to the wavelength defined by the grating and themodes of the external cavity formed thereby.

In one embodiment, gratings 220 ₁-220 _(N) are implemented assilicon-based waveguide Bragg gratings (WBGs) formed in the waveguidesimplementing the input ports of AWG 130 (which also serve as the outputports of laser array 110). For example, the WBGs may have alternatingregions of doped and undoped (or doped with different dopants) siliconresulting in alternating regions of different indices of refraction.These regions would be made with the proper width and spacing for thedesired Bragg wavelength. Alternatively, the WBGs may have alternatingregions of different materials (e.g., silicon and oxide, or silicon andpolysilicon). By using a silicon-based waveguide grating to define theoutput wavelength of each channel, wavelength drift under temperaturechanges can be reduced (e.g., DFB lasers typically have a drift of about0.1 nm/° C. compared to about 0.01 nm/° C. for a silicon-based WBG).

In yet another embodiment, the WBGs may be tunable (e.g., thermaltuning) using currently available WBG technology so that a fine-tuningof the output wavelength can be performed. Such an embodiment can beadvantageously used in WDM applications requiring precise wavelengthallocation.

The spacing between the ports of the AWG 130 and the lengths ofwaveguides in the grating region of AWG 130 are configured to multiplexthe output wavelength of each external cavity laser (defined by theBragg wavelengths of the WBGs) to the output port connected to waveguide180. Thus, AWG 130 would not direct other wavelengths that might bepresent in the optical signals output by laser array 110 to the outputport connected to waveguide 180.

One advantage of this approach is that laser gain blocks 210 ₁-210 _(N),can be fabricated as a single “bar” of substantially identical activecomponents to assemble 1×N wavelength system, operating in conjunctionwith grating array 140 to output the desired wavelengths of the WDMsystem. The bar is a piece of laser material wafer that is diced so thatit incorporates the multiple substantially identical laser gain blocks,which can reduce fabrication costs compared to singulated laser gainblocks and, in addition, simplify attachment and alignment of the lasergain blocks with waveguides 160 ₁-160 _(N).

FIG. 3 schematically illustrates an implementation of the trench-baseddetector array 120 (FIG. 1), according to one embodiment of the presentinvention. In this embodiment, each detector of detector array 120includes a tap coupler, a tap waveguide and a photodetector. Moreparticularly, as shown in FIG. 3, tap couplers 300 ₁-310 _(N) arerespectively connected to waveguides 160 ₁-160 _(N). Tap couplers 300₁-300 _(N) have tap output ports respectively connected to tapwaveguides 310 ₁-310 _(N), which in turn are respectively connected tophotodetectors 320 ₁-320 _(N). In addition, tap couplers 300 ₁-300 _(N)have “main” output ports respectively connected to waveguides 170 ₁-170_(N). In one embodiment, tap couplers 310 ₁-310 _(N) are each configuredto tap about 5% of the power of a received optical signal to itscorresponding photodetector. For example, the tap couplers can beimplemented using splitters or evanescent couplers configured to providethe desired power allocation between output ports. Further, in thisembodiment, photodetectors 320 ₁-320 _(N) are disposed on the surface ofPLC 101 (FIG. 1) as described below in conjunction with FIG. 4.

FIG. 4 illustrates in cross section an arrangement of an opticaldetector of detector array 120 (FIG. 3), according to one embodiment ofthe present invention. The other N-1 optical detectors are substantiallysimilar to the one depicted in FIG. 4. In this embodiment, an asymmetrictrench 400 is formed in PLC 101 substantially perpendicular to thedirection of light propagation in tap waveguide 3101. In particular, onesidewall 400, of trench 400 is angled of about 98 to 105 degreesrelative to the direction of propagation of the optical signal. In thisembodiment, the other sidewall of trench 400 is angled at about 45degrees relative to the direction of propagation of the optical signal,spaced about 10 to 50 microns away from sidewall 400 ₁. In addition, areflective surface 410 is formed on this sidewall. In one embodiment,reflective surface 410 is formed with a layer of metal such as Al, Au,TiW or combinations of different layers. Other embodiments use mirrorsimplemented using alternating layers of materials having differentrefractive indices (e.g., alternating layers of dielectric andsemiconductor materials).

In this embodiment, photodetector 320 ₁ is disposed above trench 400. Inone embodiment, photodetector 3201 (as well as photodetectors 320 ₂-320_(N)) are formed on a die that is attached to PLC 101 (FIG. 1) so thatthe photodetectors are aligned with trench 400. In some embodiments, aphotopolymer is used to attach the die to PLC 101.

In operation, tap coupler 300 ₁ (FIG. 3) taps a portion of the opticalsignal received via waveguide 160 ₁. In one embodiment, the opticalsignal propagated via waveguide 160 ₁ is generated by laser gain block210 ₁ (FIG. 2). The tapped portion of the optical signal is propagatedthrough tap waveguide 310 ₁ (indicated by arrow 420). This opticalsignal exits through sidewall 400 ₁ of trench 400 and is reflectedtowards photodetector 320 ₁ by reflective surface 410. Photodetector 320₁ converts the received optical signal into an electrical signal, whichis then provided to power controller 150 (FIG. 1). For example, the diecontaining photodetectors 320 ₁-320 _(N) can use flip-chip bonding, wirebonding, tape automated bonding (TAB) or other interconnect techniquesto conduct the electrical output signals from photodetectors 320 ₁-320_(N) to power controller 150.

FIG. 5 illustrates a cross section of PLC 101 (FIG. 1) showing animplementation of photodetector 320 ₁ and trench 400 (FIG. 4), accordingto one embodiment of the present invention. In this embodiment,photodetector 320 ₁ has a photodiode 500 formed on the side of the diefacing trench 400. A wire bond 510 is used to conduct the output signalgenerated by photodiode 500 to a conductive contact region 515 formed onthe surface of PLC 101. Alternatively, wire bond 510 can be omitted andflip-chip bonding techniques can be used to interconnect the diecontaining photodetector 320 ₁ to PLC 101. In this embodiment, a cap 520is attached to PLC 101 to hermetically (or semi-hermetically in otherembodiments) seal the photodetectors and prevent extraneous light fromadding noise to the photodetector output signal and to protect the chipsfrom degradation from environmental conditions (e.g., humidity).

In this embodiment, power controller 150 (FIG. 1) is external to PLC 101(e.g., a processor or microcontroller mounted on a motherboard) and isconnected to receive multiple signals from and provide power to PLC 101via an electrical connector 530. In particular, connector 530 isconnected to receive electrical signals via trances on PLC 101 (notshown) via wire bonds such as wire bond 540 used to conduct the outputsignal of photodiode 500 to connector 530 (via contact region 515).Power controller 150 can then receive the output signal of photodiode500 via a pin 550 of connector 530.

In operation, optical signal 420 (from tap coupler 300 ₁ (FIG. 3)) ispropagated through tap waveguide 310 ₁ and reflected off reflectivesurface 410 of asymmetric trench 400 (FIG. 4). The reflected opticalsignal is received directly by photodetector 500, facing the asymmetrictrench. In one embodiment, photodetector 500 converts optical signal 420into an electrical signal having a current as a function of the power ofoptical signal 420. This electrical signal is provided to powercontroller 150 (FIG. 1) via wire bond 510, contact region 515,conductive trace (not shown) formed on the surface of PLC 101, wire bond540 and pin 550 of electrical connector 530. Power controller 150 thencontrols laser gain block 210 ₁ (FIG. 2) to generate its output opticalsignal at a desired power level. The operation of power controller 150is described below in conjunction with FIG. 6 in controlling the opticalpower level of each channel.

In an alternative embodiment, laser array 110 (FIG. 1) may include alaser drive circuit (not shown) or other means to directly modulate theoutput signals from laser array 110. Power controller 150 can then beused to modulate the optical output signals as well as to equalizechannel power. In addition, in some embodiments, power controller 150can be configured to disable the laser gain block(s) of unused channels.

FIG. 5A illustrates an alternative implementation of power controller150 (FIG. 1) integrated on PLC 101 and photodetector 320, and trench 400(FIG. 4). In this alternative embodiment, photodiode 500A is formed onthe side of the die facing away from trench 400. Wire bond 510 is usedto conduct the output signal generated by photodiode 500A to an inputlead of power controller 150.

In this embodiment, optical signal 420 is propagated through tapwaveguide 310 ₁, reflected off reflective surface 410 of the asymmetrictrench, through the back side of the die to photodetector 500A. Powercontroller 150 receives the electrical output signal of photodetector500A via wire bond 510, which in turn controls laser gain block 210 ₁ aspreviously described.

FIG. 6 illustrates the operational flow of optical transmitter 100(FIG. 1) in controlling the optical power of each channel of the opticaltransmitter's output, according to one embodiment of the presentinvention. Referring to FIGS. 1 and 6, power controller 150 operates asfollows in controlling the optical power level of each channel.

As represented by a block 601, multiple optical signals are provided toPLC 101. In one embodiment, these optical signals are generated by laserarray 110, which is integrated on PLC 101. In one embodiment, laserarray 110 can be tuned to independently adjust the power level of eachoptical output signal outputted to waveguides 160 ₁-160 _(N).

The multiple optical signals, as represented by a block 603, arepropagated in waveguides formed in PLC 101. A relatively small portionof each optical signal is then tapped from the waveguides. In thisembodiment, detector array 120 taps off the small portions of eachoptical signal using tap couplers 300 ₁-300 _(N) (FIG. 3). In oneembodiment, detector array 120 taps about 5% of the power from eachoptical signal. Ideally, in that embodiment, each tapped portion ismatched with regard to percentage of power being tapped. This operationis represented by a block 605.

Then as represented by a block 607, the tapped portions of the opticalsignals are converted into corresponding electrical signals. In oneembodiment, detector array 120 converts the tapped portions intoelectrical signals using photodetectors 310 ₁-310 _(N) (FIG. 3). In someembodiments, each electrical signal has a current level that isproportional to the power level of its corresponding tapped portion.

The power level of each optical signal outputted by optical transmitter100 is controlled to a desired level, as represented by a block 609. Inone embodiment, power controller 150 controls the power levels of eachchannel to be substantially identical. More particularly, powercontroller 150 receives the electrical signals corresponding to eachchannel from detector array 120, and responsive to the current level ofeach signal, controls the corresponding laser gain block toappropriately decrease/increase its output power to achieve the desiredpower level. This process implements a control loop, as indicated by thereturn of the operational flow from block 609 back to block 601.

In some embodiments, power controller 150 is implemented using analogcircuitry. In alternative embodiments, power controller 150 isimplemented using a microcontroller or other processor. Thesealternative embodiments can be advantageously used with laser arraysthat include a laser drive circuit so that power controller 150 can beprogrammed to modulate the laser output signals as well as equalize thepower.

Embodiments of method and apparatus to generate and monitor opticalsignals and control power levels thereof in a PLC are described herein.In the above description, numerous specific details are set forth (suchas laser devices, AWGs, photodetectors, etc.) to provide a thoroughunderstanding of embodiments of the invention. One skilled in therelevant art will recognize, however, that embodiments of the inventioncan be practiced without one or more of the specific details, or withother methods, components, materials, etc. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring the description.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

In addition, embodiments of the present description may be implementednot only within a semiconductor chip but also within machine-readablemedia. For example, the designs described above may be stored uponand/or embedded within machine readable media associated with a designtool used for designing semiconductor devices. Examples include anetlist formatted in the VHSIC Hardware Description Language (VHDL)language, Verilog language or SPICE language. Some netlist examplesinclude: a behavioral level netlist, a register transfer level (RTL)netlist, a gate level netlist and a transistor level netlist.Machine-readable media also include media having layout information suchas a GDS-II file. Furthermore, netlist files or other machine-readablemedia for semiconductor chip design may be used in a simulationenvironment to perform the methods of the teachings described above.

Thus, embodiments of this invention may be used as or to support asoftware program executed upon some form of processing core (such as theCPU of a computer) or otherwise implemented or realized upon or within amachine-readable medium. A machine-readable medium includes anymechanism for storing or transmitting information in a form readable bya machine (e.g., a computer). For example, a machine-readable medium caninclude such as a read only memory (ROM); a random access memory (RAM);a magnetic disk storage media; an optical storage media; and a flashmemory device, etc. In addition, a machine-readable medium can includepropagated signals such as electrical, optical, acoustical or other formof propagated signals (e.g., carrier waves, infrared signals, digitalsignals, etc.).

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible, as those skilled in the relevant art will recognize.

These modifications can be made to embodiments of the invention in lightof the above detailed description. The terms used in the followingclaims should not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claiminterpretation.

1. An optical detection apparatus, comprising: a waveguide formed in asemiconductor substrate to propagate an optical signal received at afirst end of the waveguide; a trench formed in the waveguide having afirst sidewall and a second sidewall, the first and second sidewallsforming first and second angles with the waveguide's propagationdirection; a reflective surface formed on the second sidewall, and aphotodetector to receive an optical signal propagated in the waveguide,through the first sidewall and reflected from the reflective surface onthe second sidewall.
 2. The apparatus of claim 1, wherein thephotodetector is positioned above the trench.
 3. The apparatus of claim1, wherein the first angle ranges from about 98 to 105 degrees and thesecond angle ranges from about 43 to 47 degrees.
 4. The apparatus ofclaim 1, wherein the photodetector is attached to the semiconductorsubstrate using a photopolymer.
 5. The apparatus of claim 1, furthercomprising a cap attached to the semiconductor substrate to enclose thetrench and photodetector.
 6. The apparatus of claim 1, furthercomprising a coupler attached to the first end of the waveguide, thecoupler to receive an input optical signal and cause a portion of theinput optical signal to propagate in the waveguide.
 7. A method,comprising: propagating an optical signal through a waveguide includedin a semiconductor substrate, the waveguide having formed therein atrench having a first sidewall and a second sidewall, the first andsecond sidewalls forming first and second angles with the waveguide'spropagation direction; and receiving the optical signal using aphotodetector, the optical signal being propagated through the firstsidewall and reflected from the second sidewall to the photodetector. 8.The method of claim 7, further comprising positioning the photodetectorabove the trench.
 9. The method of claim 7, wherein the first angleranges from about 98 to 105 degrees and the second angle ranges fromabout 43 to 47 degrees.
 10. The method of claim 7, further comprisingpropagating an input optical signal to a coupler, the coupler causing aportion of the input optical signal to propagate in the waveguide. 11.An optical power control apparatus, comprising: an optical signal sourceto output an optical signal, a portion of which is propagated in awaveguide formed in a semiconductor substrate; a trench formed in thewaveguide having a first sidewall and a second sidewall, the first andsecond sidewalls forming first and second angles with the waveguide'spropagation direction, the second sidewall having a reflective layer,the portion of the optical signal propagated in the waveguide beingpropagated through the first sidewall and reflected from the secondsidewall; a photodetector to output a signal indicative of a power levelof the portion of the optical signal reflected from the second sidewall;and a controller to provide, to the optical signal source, a controlsignal responsive to the signal outputted by the photodetector, theoptical signal source to control a power level of the optical signalsource responsive to the control signal.
 12. The apparatus of claim 11,wherein the photodetector is positioned above the trench.
 13. Theapparatus of claim 11, wherein the first angle ranges from about 98 to105 degrees and the second angle ranges from about 43 to 47 degrees. 14.The apparatus of claim 11, further comprising a cap attached to thesemiconductor substrate to enclose the trench and photodetector.
 15. Theapparatus of claim 11, further comprising a coupler to receive theoptical signal and cause the portion of the optical signal to propagatein the waveguide.
 16. A method, comprising: generating an opticalsignal; propagating a portion of the optical signal through a waveguideincluded in a semiconductor substrate, the waveguide having formedtherein a trench having a first sidewall and a second sidewall, thefirst and second sidewalls forming first and second angles with thewaveguide's propagation direction; generating an output signalresponsive to a power level of the portion of the optical signal using aphotodetector, the portion of the optical signal being propagatedthrough the first sidewall and reflected from the second sidewall to thephotodetector; and controlling a power level of the optical signalresponsive to the output signal from the photodetector.
 17. The methodof claim 16, further comprising positioning the photodetector above thetrench.
 18. The method of claim 16, wherein the first angle ranges fromabout 98 to 105 degrees and the second angle ranges from about 43 to 47degrees.
 19. The method of claim 16, further comprising propagating theoptical signal to a coupler, the coupler causing the portion of theoptical signal to propagate in the waveguide.